Plasma generating system having movable electrodes

ABSTRACT

A plasma generating system is provided. The plasma generating system includes: a pair of electrodes having distal ends; an electrode holder holding the pair of electrodes; a gate having a surface on which the electrode holder is slidably mounted and adapted to be controlled by sliding the electrode holder on the surface; and a resilient member secured to the gate and adapted to generate a force to close the opening. The distal ends are adapted to move into an opening of the gate as the electrode holder slides along a direction on the surface and adapted to generate an electric arc to thereby ignite plasma in a gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/677,330, filed on Nov. 15, 2012, which claims the benefit ofU.S. Provisional Applications No. 61/561,759, entitled “Plasmagenerating system having movable electrodes,” filed on Nov. 18, 2011,which are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to plasma generators, and moreparticularly to devices having movable electrodes for igniting plasmaplumes.

2. Discussion of the Related Art

In recent years, the progress on producing plasma by use of microwaveenergy has been increasing. Typically, a plasma producing systemincludes a device for generating microwave energy and a nozzle thatreceives the microwave energy to excite gas flowing through the nozzleinto plasma. One of the difficulties in operating a conventional plasmaproducing system is making an ignition system that does not interferewith the microwave energy in the nozzle. For example, a conventionalignition system includes an electrode having a tip remaining in thenozzle after ignition. The tip heated by the microwave energy in thenozzle is eroded to suffer a shortening of its lifespan. Also, themechanism for holding the electrode may cause the leakage of microwaveenergy from the nozzle. Thus, there is a need for an ignition systemwhich ignites plasma plumes in the nozzle that reduces leaking and/orinterfering with the microwave energy in the nozzle.

SUMMARY OF THE INVENTION

In one embodiment of the present disclosure, a plasma generating systemincludes: a pair of electrodes having distal ends; an electrode holderholding the pair of electrodes; a gate having a surface on which theelectrode holder is slidably mounted and adapted to be controlled bysliding the electrode holder on the surface; and a resilient membersecured to the gate and adapted to generate a force to close theopening. The distal ends are adapted to move into an opening of the gateas the electrode holder slides along a direction on the surface andadapted to generate an electric arc to thereby ignite plasma in a gas.

In another embodiment of the present disclosure, a plasma generatingsystem includes: an electrode having a distal end; a waveguide having awall, the wall having a hole through which the electrode passes and atleast one opening for receiving a first gas therethrough; and anactuator coupled to the electrode and adapted to move the electroderelative to the wall. The distal end and the hole are configured to forma gap for receiving a second gas therethrough. Also, the distal end isadapted to move into the waveguide and generate an electric arc tothereby ignite the second gas into plasma during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of a plasma generating system inaccordance with one embodiment of the present invention, where anignition system is retracted from a cavity.

FIG. 1B shows a schematic diagram of the plasma generating system inFIG. 1A, where the ignition system is advanced into the cavity to igniteplasma in the cavity.

FIG. 1C shows a top view of an electrode holder in FIG. 1A.

FIG. 1D shows a top view of a pair of gates in FIG. 1A, where the gatesare open to pass the ignition system therethrough.

FIG. 1E shows a top view of the pair of gates in FIG. 1A, where thegates are closed to block the microwave leakage therethrough duringoperation.

FIG. 2A shows a schematic diagram of a plasma generating system inaccordance with another embodiment of the present invention, where anignition system is retracted from a cavity.

FIG. 2B is an enlarged view of the ignition system in FIG. 2A.

FIG. 2C shows a top view of a pair of gates in FIG. 2A.

FIG. 2D shows a schematic diagram of the plasma generating system inFIG. 2A, where the ignition system is advanced into the cavity.

FIG. 2E is an enlarged view of the ignition system in FIG. 2D.

FIG. 3A shows a schematic diagram of a plasma generating system inaccordance with another embodiment of the present invention, where anignition system is retrieved from a cavity.

FIG. 3B shows a schematic diagram of the plasma generating system inFIG. 3A, where the ignition system is advanced into the cavity.

FIG. 4A shows a schematic diagram of a plasma generating system inaccordance with another embodiment of the present invention, where anignition system is advanced into a cavity.

FIG. 4B shows a schematic diagram of the plasma generating system inFIG. 4A, where the ignition system is retracted from the cavity.

FIG. 4C shows a top view of a portion of the cavity wall in FIG. 4A.

FIG. 4D shows a top view of a seal in FIG. 4A.

FIG. 5A shows a schematic diagram of a plasma generating system inaccordance with another embodiment of the present invention, where anignition system is advanced into a cavity.

FIG. 5B shows a schematic diagram of the plasma generating system ofFIG. 5A, where the ignition system is retrieved from the cavity.

FIG. 6 shows a schematic diagram of a plasma generating system inaccordance with another embodiment of the present invention, where anignition system is advanced into a cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows a schematic diagram of a plasma generating system 2 inaccordance with one embodiment of the present invention. As depicted,the plasma generating system 2 includes: a plasma ignition system 10; awaveguide 9; and a gas flow tube 13 disposed in the waveguide 9 andformed of microwave transparent material, such as quartz. Microwaveenergy, which is generated by a microwave source (such as magnetron, notshown in FIG. 1A), may be transmitted through the waveguide 9. Here, theterms nozzle and gas flow tube are used interchangeably since the gasflow tube 13 operates as a nozzle through which the gas flows. In sucharrangements, the gas flow tube operates as a resonant cavity, and thus,the terms resonant cavity (or, shortly, cavity) and gas flow tube areused interchangeably. The outlet of the gas flow tube 13 may be blockedby a mesh 26, where the spacing between cords of the mesh 26 is smallenough to block the leakage of microwave energy therethrough.

The plasma ignition system 10 includes: a pair of electrodes 12 a and 12b; a pair of gates 18 a and 18 b; an electrode holder 14 slidablymounted on the slanted surfaces of the gates 18 a and 18 b and formed ofdielectric material; a pair of resilient members, such as compressionsprings, 20 a and 20 b; and a pair of stops 22 a and 22 b secured to thewaveguide 9, with each of the stop holding one end of the correspondingresilient member in place relative to the waveguide 9.

The pair of gates 18 a and 18 b is formed of microwave opaque material,such as metal, and slidably mounted on the waveguide 9 so that it canmove parallel to the top surface of the waveguide 9. FIG. 1C shows a topview of the electrode holder 14. As depicted, the pair of electrodes 12a and 12 b is secured to the electrode holder 14 and the electrodeholder 14 has a throughpass hole 23. The electrode holder 14, formed ofdielectric material, electrically insulate the pair of electrodes 12 and12 b from each other so that the electric arc 28 is generated at thedistal ends of the pair of electrodes. The electrode holder 14 may bemoved upwardly or downwardly as indicated by an arrow 16 by an actuator17. It should be apparent to those of ordinary skill in the art that theactuator 17 may be of any suitable type, such as electrical actuator. Asthe electrode holder 14 is moved downwardly, the gates 18 a and 18 bmove parallel to the waveguide 9 so that the opening or aperture 27 isenlarged while the resilient members 20 a and 20 b are being compressed.

As the electrode holder 14 further moves down, the distal ends of theelectrodes 12 a and 12 b are advanced into the cavity or gas flow tube13, as depicted in FIG. 1B. FIG. 1B shows a schematic diagram of theplasma generating system 2, where the ignition system 10 is fullyadvanced into the gas flow tube 13. Then, while the gas flows throughthe throughpass hole 23 formed in the electrode holder 14, the operatormay apply a high voltage to the electrodes 12 a and 12 b to form anelectric arc 28 at the distal ends of the electrodes. (For brevity, thehigh voltage source electrically connected to the proximal ends of theelectrodes 12 a and 12 b is not shown in FIG. 1A.) The electric arcgenerates ionized gas particles in the gas flow tube 13, which in turnignites a plasma plume 29 in conjunction with the microwave energyflowing through the gas flow tube 13. Optionally, the electrode holder14 may be interlocked with a triggering mechanism (not shown in FIG. 1B)that triggers the electric arc only when the electrode holder 14 is atits lowest position.

Upon establishing the plasma plume 29, the distal end of the ignitionsystem 10 may be retracted from the cavity by moving the ignition system10 upwardly, i.e., the ignition system 10 is returned to the position inFIG. 1A. As the ignition system 10 retracts, the size of the aperture 27decreases by the restoring force of the resilient members 20 a and 20 bso that the microwave leakage through the aperture 27 is reduced. FIG.1D shows a top view of the pair of gates 18 a and 18 b in FIG. 1A, wherethe gates are fully open to pass the electrodes 12 a and 12 btherethrough. FIG. 1E shows a top view of the pair of gates 18 a and 18b in FIG. 1A, where the gates are closed to block the microwave leakagetherethrough when the electrodes 12 a and 12 b are fully retrieved. Asdepicted, each of the gates 18 a and 18 b includes a recess 29. Asdiscussed above, the gates 18 a and 18 b are closed by the resilientmembers 20 a and 20 b when the ignition system 10 is fully retrieved.Then, the recesses 29 in the gates 18 a and 18 b will form the aperture27 so that the gas flowing through the hole 23 passes therethrough andis fed into the plasma plume 29, to thereby sustain the plasma plume 29in the gas flow tube 13.

Unlike most conventional systems, the electrodes 12 a and 12 b areretracted when the plasma plume 29 is established. If the tips of theelectrodes 12 a and 12 b were to remain in the gas flow tube 13 duringoperation, the microwave energy in the gas flow tube 13 would be pickedup by the electrodes 12 a and 12 b. Also, the tips of the electrodes 12a and 12 b would interact with plasma inside the cavity, where theinteraction would erode the tips of the electrodes. Thus, the ignitionsystem 10, by retracting the tips of the electrodes 12 a and 12 b,reduces the erosion of the electrode tips. Furthermore, the microwaveleakage through the aperture 27 is reduced as the aperture 27 is reducedduring operation. As such, the ignition system 10 enhances theefficiency of the overall system. In one embodiment, the diameter of theaperture 27 is few orders of magnitude smaller than the wavelength ofthe microwave in the waveguide 9 so that there will be no leakagethrough the gates 18 a and 18 b, further enhancing the systemefficiency.

In FIGS. 1D and 1E, the aperture 27 is formed by a pair of recesses 29in the gates 18 a and 18 b. However, it should be apparent to those ofordinary skill in the art that other suitable types of mechanism, suchas iris having multiple fingers, may be used to control the diameter ofthe aperture 27.

FIG. 2A shows a schematic diagram of a plasma generating system 30 inaccordance with another embodiment of the present invention, where anignition system 31 is retracted from a cavity surrounded by a gas flowtube 50, a waveguide 48, and two meshes 34 and 36. FIG. 2B is anenlarged view of the ignition system 31 in FIG. 2A. FIG. 2C shows a topview of the pair of gates 46 a and 46 b in FIG. 2B. As depicted, thestructure and operational mechanisms of the waveguide 48, the gas flowtube 50, the pair of electrodes 44 a and 44 b, the electrode holder 42,the pair of gates 46 a and 46 b, and the pair of resilient members 40 aand 40 b are similar to their counterparts of the plasma generatingsystem 2 in FIG. 1A, the difference being that the gas is not providedthrough the electrode holder 42 and the pair of meshes 34 and 36 areused to block the microwave leakage. Instead, the working gas isintroduced into the gas flow tube 50 through a gas passageway 47 formedin the wall of the waveguide 48. As the electrode holder 42 is movedalong the direction of an arrow 32 by an actuator 53, the pair of gates46 a and 46 b slide along the direction of an arrow 38. It should beapparent to those of ordinary skill in the art that the actuator 53 maybe of any suitable type, such as electrical actuator. The pair of gates46 a and 46 b do not include any recess similar to the recess 29 in FIG.1D, i.e., the gates 46 a and 46 b do not have any aperture when they areclosed, as depicted in FIG. 2C, since the gas is introduced through thegas passageway 47.

In another embodiment, the electrode holder 42 may include a throughpasshole as the electrode holder 14 in FIG. 1A. In such arrangements, theignition gas may pass through the hole formed in the electrode holderwhile the working gas may pass through the gas passageway 47, i.e., theignition gas, which may different from the working gas, is introducedinto the gas flow tube 50 during the initial ignition process.

FIG. 2D shows a schematic diagram of the plasma generating system 30where the ignition system 31 is advanced into the cavity or the gas flowtube 50. FIG. 2E is an enlarged view of the ignition system 31 in FIG.2D. As depicted, high voltage is applied to the electrodes 44 a and 44 bto generate an electric arc 43, which in turn ionize the gas in the gasflow tube 50. Then, the ionized gas generates a plasma plume 49 inconjunction with the microwave energy inside the waveguide 48. Uponestablishing the plasma plume 49, the ignition system 31 is retrievedfrom the gas flow tube 50, as depicted in FIG. 2A.

FIG. 3A shows a schematic diagram of a plasma generating system 70 inaccordance with another embodiment of the present invention, where anignition system 71 is retrieved from a cavity surrounded by a gas flowtube 72, meshes 76 and 78, and a waveguide 73. FIG. 3B shows a schematicdiagram of the plasma generating system 70, where the ignition system 71is moved into the cavity. The structure and operational mechanisms ofthe components of the plasma generating system 70 are similar those oftheir counterparts of the plasma generating system 30, with thedifference that the ignition system 71 is located on the bottom part ofthe plasma generating system 70, i.e., the ignition system 71 is locateddownstream of the gas flow tube 72 while the ignition system 31 islocated upstream of the gas flow tube 50. For brevity, the actuatorsimilar to the actuator 53 is not shown in FIGS. 3A and 3B.

As discussed above, the operational procedure of the plasma generatingsystem 70 is similar to that of the plasma generating system 30. Forinstance, the ignition system 71 is advanced into the cavity toestablish a plasma plume 74, as depicted in FIG. 3B. Then, the ignitionsystem 71 is retrieved from the cavity so that the gates block theleakage of microwave energy from the waveguide 73.

FIG. 4A shows a schematic diagram of a plasma generating system 80 inaccordance with another embodiment of the present invention, where anignition system 81 is advanced into a cavity surrounded by the waveguide90, a mesh 87, and a gas flow tube 96 formed of microwave transparentmaterial, such as quartz. FIG. 4B shows a schematic diagram of theplasma generating system 80, where the ignition system 81 is retrievedfrom the cavity. FIG. 4C shows a top view of a portion of the wall ofthe waveguide 90 nearby the throughpass hole 98. FIG. 4D shows a topview of the seal 88 in FIG. 4A.

As depicted in FIGS. 4A-4D, the ignition system 81 includes: anelectrode 85 movable upwardly and downwardly; a pressure plate feature84 secured to the electrode 85; a seal 88; a resilient mechanism 86,such as spring, connected to both the pressure plate feature 84 and theseal 88; a housing 82 forming an enclosed space over the waveguide 90and having a gas inlet 83, where the housing 82 may be formed ofmicrowave opaque material, such as metal. The electrode 85 iselectrically insulated from the housing 82 by an insulating member 93.In an alternative embodiment, the housing 82 and the insulating member93 are formed on one integral body of dielectric material.

To establish a plasma plume 92, the electrode 85 is moved downwardly sothat its conical tip is inside the cavity. Also, the gap between theinner rim of the hole 98 and the outer surface of the electrode 85 formsan ignition gas flow conduit, i.e., the hole 98 and the electrode 85form a gate for receiving the ignition gas. Then, high voltage isapplied to the proximal end of the electrode 85 so that an electric arc91 is formed at the distal end of the electrode 85. Here, the walls ofthe waveguide 90 may be grounded to provide a preset electricalpotential relative to the electrode 85. Alternatively, a pair ofelectrodes that is similar to the pair of electrodes 12 a and 12 b(shown in FIG. 1A) may be used in place of the electrode 85, where theelectrodes are insulated from the ground. It is noted that dielectricpads 97 may be attached to the inner surface of the waveguide 90 so thatthe arc 91 may be restricted to the region near the tip of the electrode85. Also, the dielectric pads 97 may cover sharp corners of thewaveguide 90, obviating inadvertent discharge of electric arc near thecorners.

The electric arc 91 may ionize the ignition gas introduce into the gasflow tube 96 through the hole 98. Then, the ionized gas ignites the gasinside the cavity and forms a plasma plume 92 in conjunction with themicrowave energy directed into the waveguide 90. During this stage, thepressure plate feature 84 applies a force to the seal 88 so that theseal blocks (or reduces flow through) the holes 94 formed in thewaveguide 90, as depicted in FIG. 4A.

In FIGS. 4A and 4B, the hole 98 is used as a gas flow passageway duringthe ignition process, while the holes 94 are used as a gas flowpassageway during the main plasma operation. However, depending onoperational conditions, the seal 88 may not completely close the holes94 during the ignition stage. For example, the gas may flow through boththe holes 98 and 94 simultaneously when the high voltage arc 91 isinitiated.

Once the plasma plume 92 is established, the electrode 85 is movedupwardly so that the hole 98 is blocked by the conical tip of theelectrode 85. Also, the pressure plate feature 84 moves the resilientmember 86 upwardly so that the seal 88 unblocks the holes 94, as shownin FIG. 4B. The working gas that flows through the holes 94 is fed intothe plasma plume 92 so that the plasma plume is sustained in the cavity.It is noted that the ignition gas introduced through the hole 98 may bedifferent or the same as the working gas introduced through the holes94.

As depicted in FIG. 4C, a wall of the waveguide 90 includes a pluralityof holes 94 for passing the working gas therethrough and the hole 98 forreceiving the electrode 85. In one embodiment, the diameter of each hole94 may be few orders of magnitude smaller than the wavelength of themicrowave inside the waveguide 90 so that the microwave energy does notleak through the holes 94. In FIG. 4C, only eight holes are shown.However, it should be apparent to those of ordinary skill in the artthat other suitable number of holes may be formed in the waveguide 90.

As depicted in FIG. 4D, the seal 88 has a substantially ring shape sothat it can block or restrict the holes 94 when the pressure plate 84applies a force to the seal 88. The seal 88 may be formed of material,such as metal, that is opaque to the microwave. In an alternativeembodiment, the seal 88 may be replaced by multiple valves, such asneedle valves, where each valve may control the flow through each of theholes 94.

In an alternative embodiment, the seal 88 may be directly secured to theelectrode 85 so that they can be moved as one rigid body. In thisembodiment, the pressure plate feature 84 and resilient mechanism 86 maybe eliminated from the system 80.

The electrode 85 may be moved upwardly or downwardly by an actuator 99.It should be apparent to those of ordinary skill in the art that theactuator 99 may be of any suitable type, such as electrical actuator.The actuator 99 may be located inside the housing 82.

FIG. 5A shows a schematic diagram of a plasma generating system 100 inaccordance with another embodiment of the present invention. Asdepicted, the electrode 101 may be moved so that the conical tip portionis advanced into or retrieved from the cavity formed by the waveguide102, two meshes 106 and 108; and a gas flow tube 104. The electrode 101,waveguide 102, meshes 106 and 108 may be formed of microwave opaquematerials, such as metal. FIG. 5B shows a schematic diagram of theplasma generating system 100, where the tip portion electrode of theelectrode 101 is retrieved from the cavity.

To ignite plasma in the cavity, the electrode 101 may be moved towardthe cavity by an actuator 160 so that the conical tip portion of theelectrode 101 is inside the cavity. Then, high voltage is applied to theelectrode 101 to generate an electric arc 110 at the tip of theelectrode 101. Then, the gas injected through the hole 103 in the wallof the waveguide 102, which may be a working gas, is transformed into aplasma plume 111 by use of the microwave energy directed to thewaveguide 102 according to the ignition mechanism discussed above. Toprevent electric arc inside the hole 114, the hole 114 is surrounded bya dielectric pad 116 secured to the inner surface of the waveguide 102.

When the plasma plume 111 is established inside the gas flow tube 104,the electrode 101 is moved so that the conical tip of the electrode 101is retrieved from the cavity. The conical tip of the electrode 101 isdimensioned to block the hole 114 formed in the waveguide 102 so thatthe microwave energy does not leak through the hole 114.

It is noted that the ignition gas may flow through the gap between therim of the hole 114 and the outer surface of the electrode 101 when theelectrode 101 is advanced toward the cavity. Alternatively, the ignitiongas may be injected through the hole 103. In both embodiments, theworking gas is injected through the hole 103.

As depicted in FIGS. 5A-5B, the dielectric pad 116 covers the hole 114,but it does not extend to the inner surface of the waveguide 102 thatdefines the cavity. Thus, when the electrode 101 is retracted from thecavity by the actuator 160, as shown in FIG. 5B, the conical tip portionof the electrode 101 directly contacts the waveguide 102 so that theelectrode 101 is grounded via the waveguide 102. As the electrode 101and the waveguide 102 are grounded and formed of electrically conductingmaterial, the microwave energy in the cavity would not leak through thehole 114 during operation. It should be apparent to those of ordinaryskill in the art that the actuator 160 may be of any suitable type, suchas electrical actuator.

FIG. 6 shows a schematic diagram of a plasma generating system 140 inaccordance with another embodiment of the present invention. Asdepicted, system 140 is similar to the system 100, with the differencethat the cross sectional dimension of the hole 144 formed in thewaveguide 142 varies along the longitudinal direction of the electrode141. By varying the cross sectional dimension, the impedance between theelectrode 141 and the waveguide 142 is also varied. By making theimpedance be minimum near the conical tip portion of the electrode 141,the location of the arc 150 can be limited to the area near the conicaltip portion of the electrode 141. For brevity, an actuator for movingthe electrode 141 is not shown in FIG. 6, where the actuator may besimilar to the actuator 160.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood that the foregoingrelates to preferred embodiments of the invention and that modificationsmay be made without departing from the spirit and scope of the inventionas set forth in the following claims.

What is claimed is:
 1. A plasma generating system, comprising: a pair ofelectrodes having distal ends; an electrode holder holding the pair ofelectrodes; a gate having a surface on which the electrode holder isslidably mounted and adapted to be controlled by sliding the electrodeholder on the surface; and a resilient member secured to the gate andadapted to generate a force to close an opening in the gate, wherein thedistal ends are adapted to move into the opening in the gate as theelectrode holder slides along a direction on the surface and adapted togenerate an electric arc to thereby ignite a plasma in a gas when anelectrical potential difference is applied across the pair ofelectrodes.
 2. A plasma generating system as recited in claim 1, whereinthe electrode holder includes a hole forming a passageway of the gas andwherein the electric arc is adapted to ignite a plasma in the gas.
 3. Aplasma generating system as recited in claim 2, further comprising: awaveguide for transmitting microwave energy therethrough, the gate beingslidably mounted on a surface of the waveguide; and a gas flow tubedisposed in the waveguide and having an inlet for receiving the gas thatexits the opening of the gate.
 4. A plasma generating system as recitedin claim 3, wherein the hole is dimensioned to block leakage of themicrowave energy therethrough.
 5. A plasma generating system as recitedin claim 4, wherein the gas flow tube includes an outlet, furthercomprising: a mesh disposed in the outlet of the gas flow tube andconfigured to block leakage of the microwave energy through the outlet.6. A plasma generating system as recited in claim 1, further comprising:an actuator for controlling the electrode holder.
 7. A plasma generatingsystem as recited in claim 1, further comprising: a waveguide having agas inlet for receiving a working gas and adapted to transmit microwaveenergy therethrough, the gate being slidably mounted on a surface of thewaveguide; and a gas flow tube disposed in the waveguide and having aninlet for receiving the working gas that passes through the gas inlet ofthe waveguide.
 8. A plasma generating system as recited in claim 7,further comprising: a mesh disposed in the gas inlet of the waveguideand configured to block leakage of the microwave energy through theinlet.
 9. A plasma generating system as recited in claim 7, wherein thegas flow tube has an outlet, further comprising: a mesh disposed in theoutlet of the gas flow tube and configured to block leakage of themicrowave energy through the outlet.
 10. A plasma generating system asrecited in claim 7, wherein the electrode holder includes a hole forminga passageway of an ignition gas and wherein the electric arc is adaptedto ignite a plasma in the ignition gas.
 11. A plasma generating systemas recited in claim 1, wherein the electrode holder is formed of adielectric material.
 12. A plasma generating system as recited in claim1, wherein the distal ends are adapted to move out of the opening in thegate after the plasma is ignited in the gas.